Defects in homology-directed repair (HDR)-considered the most accurate of three pathways for repairing double-strand breaks (DSBs) in DNA-can lead to genomic instability and are associated with tumor predisposition. Mutations in multiple genes involved in HDR are linked to breast cancer susceptibility including BRCA1. The mammary gland is highly influenced by its hormonal environment and undergoes significant changes during pregnancy, lactation and involution, but many questions remain about how the DNA damage response, and specifically DNA repair, is impacted by these developmental stages and during tumorigenesis. A widely used reporter for studying DSB repair by HDR is DR-GFP in which direct repeats of two defective GFP genes are induced to recombine by I-SceI endonuclease cleavage of one of the repeats. To measure HDR within tissues of an animal, we have recently generated DR-GFP containing mice that express I-SceI under the control of a tetracycline-inducible promoter, allowing for the analysis of HDR within both normal tissues and tumors. The proposed work will utilize these novel HDR reporter mice to analyze DSB repair in the developing mammary gland of normal and repair-deficient mice and in a mouse model of mammary tumorigenesis to determine how repair proficiency of mammary tissue affects breast cancer predisposition and progression. Specifically this project aims 1) to assess HDR in the mammary tissue of mice during different stages of mammary gland development to determine whether changes associated with parity affect the DSB repair response; 2) to investigate the impact of Brca1 mutations on HDR and cell fate in the mammary epithelium; and 3) to identify and characterize in vivo changes in DNA repair associated with tumor progression and response to treatment in a mouse mammary tumor model. During the K99 period (Aims 1 and 2), I will be mentored by Dr. Maria Jasin, a leader in the field of DNA DSB repair whose lab has published key papers on the roles of BRCA1 and BRCA2 in HDR. This work will shed new light on how tissue specific and genetic changes influence DNA repair processes and tumorigenesis in the mammary gland and provide valuable insight for more specific breast cancer prevention and treatment strategies in the future.